Motivation for this change

Fix #89445.

Ruby's build flag called removeReferencesToCC doesn't do it's job completely - The file rbconfig.rb still retained the reference to CC. It seems that's unavoidable if JIT support is enabled. JIT support is enabled by default and it's probable that some Gems require this feature. Since the normal ruby we are shipping ever since a while always retained this cc reference, I think it should be best to make it possible to build a ruby flavor such as rubyMinimal without cc reference and let that derivation alone while leaving all of the regular ruby's cc references as are (After @peterhoeg 's comment).

This for example allows for someone to add an override such as (tested):

self: super:

{
  ruby = super.ruby.override {
    # rubyMinimal disables too much features required by asciidoctor
    rubygemsSupport = true;
    jitSupport = false;
    useRailsExpress = false;
    zlibSupport = true;
    opensslSupport = true;
    gdbmSupport = false;
    cursesSupport = false;
    docSupport = false;
    yamlSupport = false;
    fiddleSupport = false;
  };
}

And this will allow them to not need GCC on their system for their usage of Ruby. For example, run with this PR and that overlay applied:

$ nix why-depends -f. asciidoctor gcc

And you should get:

'asciidoctor' does not depend on 'gcc'

But without this PR, (and the overlay would fail without the PR) you should get:

/nix/store/nyb3ry2yvhskrlni9dphfmj3w1ylakaw-asciidoctor-2.0.10
╚═══bin/asciidoctor -> /nix/store/06rmj52zhnb7kkp3dzvicaiddj9a8zmx-asciidoctor-2.0.10/bin/asciidoctor
    => /nix/store/06rmj52zhnb7kkp3dzvicaiddj9a8zmx-asciidoctor-2.0.10
    ╚═══bin/.asciidoctor-epub3-wrapped: …#!/nix/store/ighz4wp6bmpsm8x1lrsvwjz4m81r6a6d-ruby-2.6.6/bin/ruby.#.# This fi…
        => /nix/store/ighz4wp6bmpsm8x1lrsvwjz4m81r6a6d-ruby-2.6.6
        ╚═══lib/ruby/2.6.0/x86_64-linux/rbconfig.rb: …CONFIG["MJIT_CC"] = "/nix/store/r4l53b461b2lyclxn1pdj0n4hvbxl2l6-gcc-wrapper-9.3.0/bin/gcc".  CO…
            => /nix/store/r4l53b461b2lyclxn1pdj0n4hvbxl2l6-gcc-wrapper-9.3.0

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.